Reversible pneumopercussive soil-penetrating machines have long been known and widely used in the industry of trenchless installation and repair of pipes and cables. These machines basically comprise a tubular body, accommodating in the rear part of it an air distributing mechanism, in front part of it a sharpened chisel, and inside of it a reciprocating striker. The rear part of the chisel represents a front anvil. A tail nut in the rear part of the tubular body secures the interior assembly of the machine, keeping together the related components. A pneumatic hose is concentrically attached to the rear part of the air distributing mechanism, supplying the machine with compressed air. The air distributing mechanism controls the flow of the compressed air in a certain order, causing the striker to cyclically reciprocate inside of the tubular body. A single cycle of machine operation consists of a forward and backward stroke of the striker. During the forward mode of operation, the striker imparts an impact to the front anvil at the end of its forward stroke, resulting in an incremental penetration of the machine into the soil. The striker then begins its backward stroke, at the end of which the striker is braked by an air buffer, preventing or minimizing the impact to the rear anvil. During the reverse mode of operation, an air buffer prevents or minimizes the impact of the striker to the front anvil, while at the end of the backward stroke the striker imparts an impact to the rear anvil, causing an incremental backward displacement of the machine.
This type of reversible pneumopercussive soil-penetrating machine is described in U.S. Pat. No. 3,651,874 (March 1972); U.S. Pat. No. 3,708,023 (January 1973); U.S. Pat. No. 3,737,701 (April 1973); U.S. Pat. No. 3,744,576 (July 1973); U.S. Pat. No. 3,756,328 (September 1973); U.S. Pat. No. 3,865,200 (February 1975); U.S. Pat. No. 4,078,619 (March 1978); U.S. Pat. No. 4,214,638 (July 1980). All these patented machines have identical short-stroke air distributing mechanisms, resulting in relatively low impact energy per blow, which in turn results in relatively short incremental displacement per cycle. A detailed analysis of these patents is presented in U.S. Pat. No. 5,031,706 (July 1991) and U.S. Pat. No. 5,226,487 (July 1993) issued to Spektor (the author of the present invention).
The present inventor has developed and published analytical methodologies for optimizing cyclic soil-working processes with respect to minimum energy consumption. These methodologies show that minimum energy consumption can be achieved at a certain optimum displacement per cycle (see Minimization of Energy Consumption of Soil Deformation, Journal of Terramechanics, 1980, Volume 17, No. 2 pages 63 to 77; Principles of Soil-Tool Interaction, Journal of Terramechanics, 1981, Volume 18, No. 1, pages 51 to 65; Motion of Soil-Working Tool Under Impact Loading, Journal of Terramechanics, 1981, Volume 18, No. 3, pages 133 to 156; Working processes of Cyclic-Action Machinery for Soil Deformation-Part I, Journal of Terramechanics, 1983, Volume 20, No. 1, pages 13 to 41; Minimum Energy Consumption of Soil Working Cyclic Processes, Journal of Terramechanics, 1987, Volume 24, No. 1, pages 95 to 107). Based on these investigations, the performance of the existing vibratory soil working machines can be evaluated by comparing their displacement per cycle with the respective optimum displacement. On analyzing these comparisons, it became apparent that the existing machines could only develop displacements per cycle that are significantly shorter than the respective optimum displacements. This results in relatively high energy consumption and relatively low productivity (average velocity). In order to improve the efficiency of these machines it is necessary to considerably increase the impact energy of the striker. This is achievable trough a significant increase of the stroke of the striker, while keeping the nominal air pressure unchanged (because the nominal air pressure of 100 psi is standard among the vast majority of industrial air compressors). However, the existing machines incorporate a short-stroke air distributing mechanism, and it is inherently impossible to significantly increase the stroke of their strikers. Based on all these considerations, the author of the present invention developed a reversible pneumopercussive soil-penetrating machine that is characterized by a long-stroke air distributing mechanism, which is described in U.S. Pat. No. 5,311,950 issued to Spektor in May 1994. This machine, due to its long-stroke air distributing mechanism allowed improved performance, however several structural complexities of this machine increased its cost while limiting its efficiency. In order to overcome these disadvantages, the author of the present invention developed a monotube reversible pneumopercussive soil penetrating machine with stabilizers, which is described in U.S. Pat. No. 5,457,831 issued to Spektor in November 1995. Laboratory and field testing of numerous machines based on this patent demonstrated a considerable increase of the efficiency of the machine with significantly reduced cost. However, extensive testing of these machines revealed several severe disadvantages that prevented the implementation of these machines.
The most critical disadvantage is associated with the fatigue failure of the spring that exerts an outward thrust on the stroke control valve and the follower and the spring that exerts an outward thrust on the relief valve. These failures occurred in most of the machines, and it was necessary to frequently replace these springs as preventive maintenance against failure. The appropriate engineering calculations associated with this specific case show that the fatigue failure could be avoided with a significant increase in length of the springs, but this would require a respective increase of the length, weight, complexity, and cost of the machine, along with a significant decrease of its efficiency.
Another disadvantage of the considered machine is related to the need of the mentioned follower and associated components such as the spacer and a separated rear anvil. First of all, the structural design of these components makes it extremely difficult to extract a small fragment of a broken spring. This fragment may cause a moving part of the machine to jam, resulting in failure of the machine. Secondly, securing the rear anvil by means of press fit and pins increases the manufacturing cost of the machine. Thirdly, fabricating and assembling all these associated parts also increases the cost of the machine.
Still another disadvantage of the considered machine is that the frictional force between the inner surface of the rear valve chest and the O-ring on the relief valve changes from machine to machine due to the manufacturing tolerances, causing a need for increased pressure in the reduced (low) pressure line, resulting in decreased efficiency of the machine.
One more disadvantage of the considered machine is related to the method of retracting a failed machine from the hole. According to this method a special attachment should be mounted to the chisel of an identical machine. This attachment should engage with the tail of the failed machine, which will be retracted by reversing the second machine. Firstly, the inherent gaps between the movable parts of the attachment cause the attachment to tilt down and shave the soil on the bottom of the hole. This may sometimes prevent the engagement of the attachment with the failed machine. Secondly, the need of a special attachment leads to additional cost and maintenance.
Still one more disadvantage of the considered machine is the absence of an option to replace the rear chest assembly (comprising the rear chest, the step-bushing, the relief valve with its O-ring and spring) with a flange having appropriate air ducts. This type of machine, having a significantly reduced cost, would have many specific applications, which will be discussed later.
Another disadvantage of the considered machine as well as the existing machines is that, during operation, the entire outer surface of the tubular body is in permanent contact with the soil, developing an essential lateral friction resistance, thus decreasing the efficiency of the working process.
The machine according to the present invention is free of all these disadvantages and is characterized by an essentially higher efficiency and a significantly lower manufacturing cost. Extensive testing of these machines in laboratory and field conditions demonstrated their numerous advantages in comparison with the considered machines. It should be emphasized that the machines according to the present invention possess a very high reliability at a drastically minimized maintenance.